﻿FOGS 
  AND 
  CLOUDS 
  — 
  HUMPHREYS. 
  205 
  

  

  crests 
  or 
  tops 
  of 
  these 
  waves 
  are 
  cooled 
  by 
  expansion, 
  having 
  risen 
  

   above 
  their 
  previous 
  level, 
  and 
  their 
  troughs 
  or 
  bottoms 
  warmed 
  

   by 
  compression. 
  Hence, 
  when 
  the 
  under 
  layer 
  is 
  practically 
  satu- 
  

   rated, 
  each 
  crest, 
  because 
  of 
  this 
  cooling, 
  is 
  cloud-capped, 
  and 
  each 
  

   trough 
  clear. 
  

  

  It 
  is 
  particularly 
  interesting 
  to 
  note 
  that 
  although 
  each 
  billow 
  

   cloud 
  maintains 
  its 
  identity 
  and 
  position 
  in 
  the 
  series 
  of 
  waves 
  as 
  

   long 
  as 
  it 
  exists 
  at 
  all 
  — 
  for 
  hours 
  it 
  may 
  be 
  — 
  nevertheless 
  the 
  

   actual 
  particles 
  of 
  which 
  it 
  consists 
  at 
  any 
  instant 
  have 
  only 
  the 
  

   briefest 
  duration, 
  there 
  being 
  continuous 
  condensation 
  on 
  the 
  

   rising 
  or 
  windward 
  side 
  of 
  the 
  billow 
  and 
  equivalent 
  evaporation 
  

   in 
  the 
  descending 
  or 
  leeward 
  portion. 
  

  

  Furthermore, 
  while 
  the 
  billow 
  cloud 
  progresses 
  steadily 
  across 
  

   the 
  sky, 
  its 
  velocity 
  is 
  neither 
  that 
  of 
  the 
  upper 
  stratum 
  nor 
  of 
  the 
  

   lower, 
  any 
  more 
  than 
  the 
  velocity 
  of 
  a 
  water 
  wave 
  is 
  either 
  that 
  of 
  

   the 
  wind 
  that 
  is 
  producing 
  it 
  or 
  of 
  the 
  water 
  in 
  which 
  it 
  occurs. 
  

   Wind 
  velocity, 
  therefore, 
  can 
  not 
  be 
  determined 
  by 
  measurements 
  

   on 
  clouds 
  of 
  this 
  kind; 
  nor, 
  of 
  course, 
  by 
  attempted 
  measurements 
  

   on 
  any 
  of 
  the 
  stationary 
  types, 
  such 
  as 
  those 
  mentioned 
  below. 
  

  

  Lenticular 
  cloud. 
  — 
  The 
  lenticular 
  cloud 
  (pi. 
  17, 
  tig. 
  2; 
  pi. 
  18, 
  figs. 
  

   1 
  and 
  2; 
  pi. 
  19; 
  and 
  pi. 
  20, 
  fig. 
  1) 
  is 
  only 
  the 
  cloud 
  cap 
  to 
  a 
  

   stationary 
  or 
  nearly 
  stationary 
  air 
  billow 
  produced 
  as 
  a 
  rule 
  by 
  

   the 
  flow 
  of 
  the 
  wind 
  over 
  an 
  uneven 
  surface. 
  It 
  is 
  especially 
  

   common 
  among 
  high 
  peaks 
  and 
  rugged 
  mountains. 
  Its 
  material, 
  

   like 
  that 
  of 
  the 
  crest 
  cloud, 
  is 
  in 
  rapid 
  change 
  — 
  condensation 
  on 
  

   the 
  windward 
  side 
  and 
  evaporation 
  to 
  the 
  leeward. 
  Hence 
  it 
  is 
  

   shaped 
  like 
  a 
  double 
  convex 
  lens, 
  the 
  suggestive 
  origin 
  of 
  its 
  name, 
  

   thickest 
  in 
  the 
  middle 
  and 
  thinning 
  away 
  to 
  nothing 
  on 
  either 
  side. 
  

  

  As 
  implied 
  above, 
  this 
  is 
  a 
  stationary 
  cloud, 
  and 
  hence 
  one 
  of 
  the 
  

   several 
  that 
  can 
  not 
  be 
  used 
  to 
  measure 
  wind 
  velocities. 
  

  

  Crest 
  cloud. 
  — 
  The 
  crest 
  cloud 
  (pi. 
  20, 
  fig. 
  2) 
  is 
  caused 
  by 
  the 
  up- 
  

   ward 
  deflection, 
  and 
  consequent 
  cooling 
  by 
  expansion, 
  of 
  humid 
  

   winds 
  by 
  a 
  long 
  mountain 
  ridge, 
  whose 
  crest 
  it 
  commonly 
  covers, 
  

   whence 
  its 
  name, 
  and 
  whose 
  sides 
  it 
  often 
  gracefully 
  drapes. 
  Occa- 
  

   sionally, 
  however, 
  it 
  forms 
  slightly 
  above 
  and 
  a 
  trifle 
  to 
  the 
  leeward 
  

   of 
  the 
  ridge 
  along 
  the 
  topmost 
  (hence 
  coolest) 
  portion 
  of 
  the 
  de- 
  

   flected 
  wind 
  current. 
  In 
  either 
  case 
  the 
  cloud 
  is 
  permanent 
  in 
  posi- 
  

   tion 
  only, 
  being 
  continuously 
  created 
  (condensed) 
  on 
  the 
  wind- 
  

   ward 
  or 
  ascending 
  and 
  cooling 
  side, 
  and 
  destroyed 
  (evaporated) 
  on 
  

   the 
  lee 
  or 
  descending 
  and 
  warming 
  side. 
  

  

  The 
  best 
  known 
  example, 
  perhaps, 
  of 
  this 
  interesting 
  cloud 
  is 
  the 
  

   celebrated 
  " 
  Table 
  Cloth 
  " 
  of 
  Cape 
  Town 
  pride, 
  spread 
  by 
  the 
  south- 
  

   easterlies 
  over 
  the 
  top 
  and 
  down 
  the 
  sides 
  of 
  Table 
  Mountain. 
  

  

  